Radiotherapy is an indispensable strategy for cancer treatment. About 60-70% of patients with malignancies receive radiation therapy which can cure many tumors and the cure rates of radiotherapy on early tongue cancer, nasopharynx, laryngeal cancer, esophageal cancer and cervical cancer are about 90% [Hogle W P, Semin Oncol Nurs, 22 (4): 212-220, (2006)]. However, when killing the tumor, radiotherapy will cause off-target effect on normal tissue (including non-cancerous tissue inside the radiation shield and distant tissues such as bone marrow), which is the major reason limiting the efficacy of radiotherapy. Although measures including the equipment updating to improve radiotherapy accuracy, cooperative usage of radiosensitizers and combination with chemotherapy have been tried, the results remain unsatisfactory. The main question turns out to be the incapability of normal tissues to tolerate radiation-induced toxicity, which prevents higher dose of radiotherapy in clinical applications. Radiation-induced lung injury is the most common clinical complications post-radiotherapy. In severe cases, it may endanger the patient's life, especially who suffer from lung tumor, esophageal tumor, breast tumor and mediastinal tumor.
Radiation-induced lung injury (RILI) consists of radiation pneumonitis in the early stage and radio-pulmonary fibrosis in the late stage. Such injury not only undermines the control of tumor, but also seriously affects the quality of life of the patients. Respiratory failure is one of the leading causes of death in RILI. In addition, local hypoxia, inflammatory response, angiogenesis, local microenvironmental changes and immunosuppression caused by RILI will promote tumor recurrence, invasion and metastasis [van den Brenk, H A et al, Br J Radiol, 47 (558): p. 332-336, (1974)]. Thus, it is particularly important to manage RILI in clinic, but the lack of effective drug leading to empirical use of high-dose glucocorticoid and anti-inflammatory drugs. These measures not only fail to improve the therapeutic effect of radiotherapy, but also cause many side effects, such as immunosuppression after long-term high-dose usage of glucocorticoids. Additionally, pulmonary immunosuppressive microenvironment spurs the risk of tumor recurrence and metastasis. It has been an urgency for tumor radiotherapy to discover a drug that can both prevent/treat RILI and reduce tumor recurrence or metastasis.
It is generally accepted that the tumor-killing effect of radiotherapy is due to radiation-induced DNA damage and the production of free radicals inside tumor cells [Muruve D A et al, Nature, 452 (7183): 103-107, (2008)]. Subsequently, DNA fragments and reactive oxygen species (ROS) trigger the inflammation, during which the activated macrophages synthesize and secrete a large amount of inflammatory cytokines, such as TNF-α, IL-1β, IL-8, etc. High levels of TNF-α and fibronectin together can cause the initial acute pneumonitis, which can also promote the proliferation of fibroblasts and stimulate fibroblasts to secrete excess collagen at the same time. Radiation-induced oxidative damage in the pulmonary capillary endothelial cells, including DNA breakage, cell death, and the increase of ROS/RNS, causes the accumulation, transcription and up-regulated activity of HIF in tumor cells [Lerman, O Z, et al., Blood, 116 (18): 3669-3676, (2010)]. Under hypoxia, vascular endothelial cells (ECs) produce a large amount of chemokine SDF, which binds to CXCR4 and recruits BMDCs to inflammatory lesions [Du, R., et al., Cancer Cell, 13 (3): 206-220, (2008)]. A big body of studies have shown that BMDCs are crucial for the formation and growth of tumor neovascularization. The changes of the microenvironment provide a favorable condition for tumor recurrence and metastasis.
The sustained and persistent immune response after the radiotherapy will cause chronic inflammation, which will then initiate the process of tissue remodeling and repair. The repair of normal tissue and infiltration of inflammatory cells promote the transformation of myofibroblasts and the secretion of transforming growth factor (TGF-β) and connective tissue growth factor (CTGF). Wherein TGF-β is a multifunctional cytokine involved in the regulation of cell proliferation, differentiation and extracellular matrix secretion, which is the most important cytokine in the pathogenesis of fibrosis [Sime, P J, et al., Am J Pathol, 153 (3): 825-832, (1998).]. However, the tissue repair process after radiotherapy is distinct from the repair process of normal tissue. Radiotherapy causes the dysfunction of vascular endothelial cells, followed by vascular lesions formation, and hypoxia-induced irreversible tissue proliferation, which finally results in radiation-induced fibrosis in the late stage.
Clinical studies have demonstrated that despite typical glucocorticoids such as dexamethasone, can effectively alleviate early radiation-induced pneumonia due to the immunosuppressive property, they potentially ablate the efficacy of radiotherapy and may worsen radiation-induced pulmonary fibrosis [Laura S., et al., Immunity, 42 (4), 767-777, (2015)]. Other studies have shown that a diet containing flavonoids have significant antioxidant effect. Polyphenols, vitamin E, etc. also have significant radioprotective effects. However, they are finally abandoned, due to the fact that the protective effects of these antioxidants on radiotherapy often results in the counteraction against radiotherapy [Sakamoto K, et al., Br J Radiol. 46 (547): 538-540, (1973)] or tumor recurrence. [Prasad K N, et al., Nutr Cancer.—19, (1996)].
The existing drugs for the prevention and treatment of RILI have protective effects, but they tend to ablate the therapeutic effect of radiotherapy, which leaves a dilemma to clinic. Therefore, a drug which can prevent RILI without affecting the efficacy of radiotherapy is urgently needed in clinic.
Flavonoids (the general structural formula shown in Formula 1) have antioxidant and free radical scavenging effects. Such compounds can terminate the free radical chain reaction by reacting phenolic hydroxyl groups with free radicals to generate more stable semicarbazide-type free radicals. Domestic and foreign scholars have found that soy isoflavones in flavonoids, such as genistein, have significant antioxidant and free radical scavenging effects and have a certain protective effect on RILL But genistein promotes metastasis in some hormone-related tumors. This study found many limitations in clinical applications of genistein in the future [El Touny L H, et al., Cancer Res. 69 (8): 3695-703, (2009)].

Formula 1: the general structural formula of flavonoids.
Naringin (the structure formula shown in Formula 2, naringin), also known as naringoside and hesperetin, with a molecular formula of C27H32O14 and a molecular weight of 580.53, mainly exists in the peel and pulp of Rutaceae plants including grapefruit, mandarin orange and orange. Naringin is also the main active ingredient of Chinese herbal rhzizoma drynariae, citrus aurantium, fructus aurantii and exocarpium citri rubrum.

Formula 2: the general structural formula of naringin.
Naringenin (naringenin, the structure formula shown in Formula 3) is the aglycone of naringin, and also the structure part of the naringin exerting its pharmacological effects. The naringenin has a molecular formula of C15H12O5 and a molecular weight of 272.25, with antibacterial, anti-inflammatory, spasmolytic and diuretic effects [12-14]. The structural formula of flavonoid and naringenin and naringin is as follows:

Formula 3: the general structural formula of naringenin.
B ring of the flavonoid is the main active site responsible for the antioxidant and free radicals scavenging effects. When there are ortho-hydroxyls in the ring, the antioxidant capacity will be greatly enhanced. The 2, 3 double-bond is preferable to the formation of a more stable free radical after the B ring loses electrons. The 4-carbonyl can form a hydrogen bond with the ortho-hydroxyl to make the free radical intermediates more stable. The 3,5-hydroxyl belongs to the synergizing phenolic hydroxyl. Thus, naringenin and naringin do not have significant antioxidant effects as soy isoflavones do [M. J, et al., Current Drug Metabolism, 10, 256-271, (2009)].
Based on systematic research, we found that naringenin and naringin exert the anti-fibrosis and anti-metastasis effects by regulating the secretion of TGF-β [CN 101322700 B; Du G J, et al., Cancer Res 69 (7): Lou C J, et al, PLOS one, 7 (12), e50956, (2012)]. Moreover, naringenin and naringin regulate the release of immune-related inflammatory cytokines to prevent radiation-initiated tumor recurrence and metastasis without affecting the therapeutic effect of radiotherapy on tumor.